A CMM machine, in the most simple terms, is a device used to measure the geometry of an object or part. A variety of probes are used to "touch" the surface of the parts at different points, which creates an eletronic pulse. This is then fed into software which maps outs these pulses and forms a dimensional drawing of the object. Dependant on the size of the object, this could be repeated to gather hundreds or thousands of points.
Accuracy and Reliability: In comparison to hand held gauges and other digital measuring techniques, the CMM uses CAD comparisons to increase accuracy to a tighter tolerance level.
Versatility: The array of probes and techniques available on the CMM allow for complex parts, which would normally be difficult to measure, to be checked relatively easily.
Repeatability: The CMM allows for repeat measurements of identical parts. Once the object has been mapped once, the program can be stored to check the same part again and again.